Thin films of cholesteric liquid crystals have the unique property of scattering light of various colors depending on the temperature of the liquid crystal. Normally, a red color is observed at the lower end of the temperature range and the color shifts toward the blue as the film is heated. The temperature at which the red first appears and the total temperature range through which color is observed is a function of the particular compound or mixture of compounds making up the liquid crystal, including, in the latter case, the proportions of the compounds in the mixture. When such films are coated on suitable substrates, the temperature of the substrate can be determined by the color of the film. Temperature gradients of less then 0.1.degree. C. can be resolved in this manner. This property has been utilized for a variety of applications, such as, the thermal mapping of surfaces of the human body and for detection of discontinuities in industrial or constructional members.
Temperature sensitive, micro-encapsulated liquid-crystal compositions are known in the art of thermometers or thermographic plates.
The compositions in this art have included numerous cholesteryl esters.
The color-temperature response of many cholesteryl esters has been characterized. Combinations of esters may be used to measure temperatures from -20.degree. C. to 250.degree. C. Cholesteric substances, which have a color reaction in this range, are usually binary, tertiary, or ternary mixtures of cholesteryl esters. Temperature ranges and compositions of various mixtures are used for thermal imaging. Compositions in the art have suffered from lack of reproducibility, sensitivity, precision accuracy and brightness, in part because there was no appreciation in the art of the important parameters of component purity, capsular size and capsular size distribution which affected packing densities resulting in poor calibration.